1. Field of Invention
The invention relates to a process for preparing curable polyolefins and is particularly directed to a process for preparing curable polyolefin polymers in the form of discrete, dry, free-flowing particles.
2. Prior Art
In U.S. Pat. No. 3,731,173, issued May 29, 1973, reference is made to Japanese Pat. No. 501,087, granted Sept. 18, 1964, for a method of blending a polyolefin with a curing agent without the excessive thermal history and other disadvantages of the earlier process of using a Banbury.TM. mixer or like blending mill or kneading-type mixer. In this process, a curing agent, for example, dicumyl peroxide, is added to polyolefin powder or granules and the resulting mixture stirred by means of a drum tumbler, blender, or the like, at a temperature at which the curing agent is liquid but below the melting point of the polyolefin. This process, according to U.S. Pat. No. 3,736,173, while avoiding the disadvantages of the Banbury.TM. mixer process, is ineffective to produce dry, free-flowing, polymer granules or powder so that the resulting product is not suitable for handling by mechanized and pneumatic transfer equipment.
U.S. Pat. No. 3,736,173 discloses a process which purports to have avoided these disadvantages and to produce a dry, free-flowing product. In this process, the particles of polymer are blended with a curing agent in a "high speed" mixer, such as a Henschel mixer, operating at a speed sufficiently high that the surface of the polyolefin granules is molten in a moment in which portions of the granules come into friction and collision with each other, with the inside wall of the mixing tank, and with the stirring blades. The patent states further that it is impossible at all to obtain such high speed stirring by means of a low speed stirring apparatus such as a drum tumbler which is driven at most at 60 r.p.m. and that the desired result is obtainable by use of a high intensity, vortical-action mixer, or like high speed mixer. This process has the disadvantages that the surface of the granules must be made molten, thus risking agglomeration, and that the process is not suitable for polyolefin polymers which, either due to admixing with filler or the like, or blending with other polymeric material, have a low concentration of polyolefin polymer. It also has the disadvantage that the polymer particles are subjected to attrition by collision with high velocity stirring blades.